Evaluating potential effects of exotic freshwater fish from incomplete species presence-absence data

Biological Invasions 4: 101–113, 2002.© 2002 Kluwer Academic Publishers. Printed in the Netherlands.

Original article

Evaluating potential effects of exotic freshwater fish fromincomplete species presence–absence data

Miguel Pascual1,∗, Patricio Macchi2, Javier Urbanski3, Fernando Marcos4,Carla Riva Rossi1, Mauro Novara1 & Patricia Dell’Arciprete1

1Centro Nacional Patagonico-CONICET, (9120) Puerto Madryn, Chubut, Argentina; 2Centro RegionalUniversitario Bariloche, Universidad Nacional del Comahue, Bariloche, Rıo Negro, Argentina; 3Centro deEcologıa Aplicada del Neuquen, Junın de los Andes, Neuquen, Argentina; 4Subsecretarıa de Pesca y ActividadesPortuarias, Rıo Gallegos, Santa Cruz, Argentina; ∗Author for correspondence (e-mail: [email protected];fax: +54-2965-451543)

Received 9 November 2001; accepted in revised form 9 May 2002

Key words: distribution, impacts, introduced fish, native freshwater fish, Patagonia

Abstract

Many freshwater ecosystems and biotas around the world are threatened with extinction. Freshwater fishes, forexample, are the most endangered vertebrates after amphibians. Exotic fish are widely recognized as a majordisturbance agent for native fish. Evaluating the ecological effects of invaders presents many challenges and theproblem is greatly augmented in parts of the world where the native fauna is poorly known and where exotic speciesare commonplace. We use the fish community of Patagonia, a small and distinct native biota dominated by exoticsalmonids, as a case study to ask: what can we learn about the effects of exotic fish species from fragmentary orpartial data and how do such data point the way to what needs to be learned? We review the available data andliterature on the distribution and status of native and introduced fish. We compile a novel regional presence/absencespecies database, build fish distribution maps, describe distribution patterns of native and exotic species, and identifycritical information voids. A comparative review of literature from Patagonia and Australasia, where a similar nativeand exotic fish fauna is found, helps us to identify research priorities and promising management strategies for theconservation of native fish fauna. We conclude that the main challenge for fish conservation in Patagonia is to identifymanagement strategies that could preserve native species while maintaining the quality of salmonid fisheries.

Introduction

Throughout the world, whole freshwater ecosystemsand biotas are threatened with extinction on a grandscale (Olson et al. 1998; Ricciardi et al. 1999). Byand large, however, freshwater biodiversity has beenseriously neglected (Allan and Flecker 1993; Leidy andMoyle 1998; Saunders et al. 2002). Conservation biol-ogy has chiefly concentrated on terrestrial biodiversity,which is readily observed and with which humans aremore familiar (Olson et al. 1998). The lesser attentionpaid to freshwater ecosystems has also been attributed

to the fact that freshwater conservation requires greaterattention to complex processes, such as large scaledynamics, complex interactions and linkages with ter-restrial systems, which are poorly understood, difficultto study and politically challenging (Olson et al. 1998).

The problem of freshwater conservation is well illus-trated by the case of fish species. More than one-halfof all vertebrates, about 24,600 species, are fish, 41%of which are exclusively freshwater (Leidy and Moyle1998). Freshwater fishes are thought to be the mostendangered vertebrates after amphibians (Saunderset al. 2002). The fish fauna of North America, arguably

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one of the best known in the world, presents a com-pelling example of the vulnerability of freshwater fish.Miller et al. (1989) identify 27 species that becameextinct during the last century. About one-third of theexisting 1174 freshwater fish species are threatened(Leidy and Moyle 1998). The chief factors affectingfish in watersheds around the world are habitat loss andspecies introductions, followed by chemical pollution,hybridization, and overharvesting (Allan and Flecker1993; Saunders et al. 2002).

The issue of introduced fish species deservesa special treatment. Freshwater fish species havebeen purposely and extensively transferred aroundthe world. Only 10% of successful fish introductionshave been unintended transfers (Welcomme 1984).The main motivations for introductions have beenesthetical, recreational and, more recently, to pro-mote aquaculture (Allan and Flecker 1993); in fact,many successful transfers gave rise to well-establishedrecreational fisheries and aquaculture activities. Thetransfer of exotic fish is an ongoing and very activeprocess worldwide, as intensive aquaculture basedon non-native species is promoted in many parts ofthe world as an instrument of economic develop-ment (Anonymous 1990). Accidental introductions dueto the escape/release of aquarium fish are likely toincrease; as many as 6000 species may ultimately beof interest to the pet trade (Allan and Flecker 1993).

Evaluating the ecological effects of invaders presentsmany challenges and there is a lack of generaliza-tions regarding the development of measures of impactfrom empirical examples or from theoretical reasoning(Parker et al. 1999). The problem is greatly augmentedin parts of the world where the native fauna is poorlyknown and where exotic species are commonplace.Given these circumstances, what can we neverthelesslearn from fragmentary or partial data – and, in partic-ular, how do such partial data point the way to whatneeds to be learned?

This paper deals with the native fish fauna of lakesand rivers in Patagonia, the southernmost region ofSouth America, which is poorly known and is presentlydominated by several species of salmonids, the exclu-sive focus of significant recreational fisheries (Leitch1991). Patagonian fish have been largely absent fromconservation agendas of provincial and federal gov-ernments, as well as of NGOs [Pascual et al. (1998),but see Olson et al. (1998) for a general conserva-tion assessment of freshwater biodiversity in LatinAmerica].

A review of the status of native and introduced fishon a regional scale appears to be a fitting initial step.We start by compiling presence/absence data on bothnative and introduced fish in the Argentinean portionof Patagonia, building what we believe is the mostupdated species distribution database for the region.We then analyze available evidence of impacts by intro-duced fish, including information from New Zealandand Australia where fish assemblages share some sim-ilarities with those of Patagonia. Finally, we criti-cally examine all the information presented to identifyapproaches that appear to be more promising in focus-ing future research on the effects of exotic species,and to identify species and conditions that appear todemand particular attention within future conservationplans for Patagonian fish.

Materials and methods

River basins of Patagonia

Argentinean Patagonia is an area of over 800,000 km2,comprised of 5 of the 23 provinces of Argentina. Itextends from latitude 37◦–55◦ S, comprising 28% ofthe national territory. The hydrographic network ofPatagonia consists of 32 major river basins (Figure 1).Fifteen watersheds drain from the Andes across thePatagonian steppe into the Atlantic Ocean, 9 are sharedwith Chile, draining across the Andes into the PacificOcean, 1 basin has mixed Pacific and Atlantic drainage,and 7 are endorheic.

Our examination of the fish fauna of Patagonia isintended as groundwork, large-scale analysis and doesnot consider the heterogeneity within and among riverbasins of the region. It must be recognized, however,that Patagonia is environmentally and ecologicallydiverse. Olson et al. (1998) recognized three majorhabitat types for the region (Wet Region Rivers andStreams, Xeric Region Rivers and Streams, and XericRegion Endorheic Basins), dividing it into two distinctecoregion complexes (Southern Chile Complex andPatagonia Complex).

The fish fauna of Patagonia

Indigenous faunaWhile South America as a whole has the highest fresh-water fish species diversity in the World, with 40%of all known continental fish species (Lagler et al.

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Figure 1. Patagonia, its provinces, and major river basins. Atlantic river basins: 1 Colorado, 2 Negro, 3 Chubut, 4 Chico, 5 Santa Cruz, 6 Coyle,7 Gallegos, 8 Grande, 9 Ewan, 10 San Pablo, 11 Lainez, 12 Irigoyen, 13 Bueno, 14 Lopez, 15 Moat. Pacific river basins: 16 Lacar, 17 Manso,18 Puelo, 19 Futaleufu, 20 Corcovado, 21 Pico, 22 Pueyrredon, 23 San Martın, 24 Fagnano. Endorheic basins: 25 Arroyo Valcheta, 26 LagunaBlanca, 27 Laguna Carrilaufquen, 28 Laguna Ne Luan, 29 Lago Cardiel, 30 Lago Strobel, 31 Senguerr. Mixed Pacific–Atlantic basins: 32Buenos Aires Lake.

1962), the indigenous fauna of Patagonia is composedof only 20 species (Table 1). Albeit species poor, thisfish fauna is distinct, including species of assortedorigins: neotropical, circumpolar, and a significantnumber of endemisms (Arratia et al. 1983).

Only four of the 20 known species are widely dis-tributed throughout South America, with the remainderbeing restricted to the northern fringe in Patagonia: twoCharacidae (Astianax eigenmaniorum and Cheirodoninterruptus), one Anablepidae (Jenynsia lineata), andone Poeciliidae (Cnesterodon decenmaculatus). Two

species are widely distributed throughout circumpo-lar regions: Galaxias maculatus, the most widelydistributed Galaxiidae (McDowall 1971), and thePetromyzontidae Geotria australis (Nelson 1994),found also in Australia, New Zealand, Tasmania andChile.

The remaining 14 species are restricted to southernSouth America. Three species of Galaxiids, Aplochitonteniatus, A. zebra and Galaxias platei, are foundon both sides of the Andes, from Central Chile toPatagonia, all the way south to Tierra del Fuego

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Table 1. Freshwater fishes of Argentinean Patagonia. Indigenous species are shown in the left column (presentdistribution for families and species indicated with letters: P – Patagonia, CH – Chile, CW – central-western Argentina,C – circumpolar, S – South America, A – Americas). The right column shows introduced species (origin and year offirst introduction into Patagonia).

Indigenous species Exotic species

Order Siluriformes EstablishedFamily Diplomystidae (A) Order SalmoniformesDiplomystes viedmensis (P), otuno Family SalmonidaeD. mesembrinus (P), otuno Salmo trutta (Europe, 1909), brown troutD. cuyanus (P + CW), otuno S. salar (USA, Canada, 1904), Atlantic salmon

Oncorhynchus mykiss (USA, 1904), rainbow troutFamily Trichomycteridae (A) O. tshawytscha (USA, 1904; Chile, 1984), chinook salmonHatcheria macraei (P + CW), bagre del torrente O. masou (Japan, 1987), cherry salmonTrichomycterus areolatus (S + CW), bagre pintado Salvelinus fontinalis (USA, 1904), brook trout

S. namaycush (USA, 1904), lake troutOrder OsmeriformesFamily Galaxiidae (C) Order AtheriniformesGalaxias maculatus (C), small puyen Family AtherinopsidaeG. platei (P + CH), large puyen Odontesthes bonariensis (Arg, 1939), silversideAplochiton taeniatus (P + CH), peladillaA. zebra (P + CH), peladilla listada Order Cypriniformes

Family CyprinidaeOrder Perciformes Cyprinus carpio (Brazil,?), common carpFamily Percichthydae (G)Percichthys colhuapensis (P), largemouth perch Order SiluriformesP. trucha (P + CH), smallmouth perch Family CallichthydaeP. altispinnis (P), largespine perch Corydoras sp. (?), armored catfishP. vinciguerrai (P), perch

Non-establishedOrder Petromyzontiformes Order SalmoniformesFamily Petromyzontidae (C) Family SalmonidaeGeotria australis (C), pouched lamprey O. kisutch (USA, 1904), coho salmon

O. nerka (USA, 1904), sockeye salmonOrder Characiformes Coregonus clupeaformis (USA, 1904), lake whitefishFamily Characidae (A)Astyanax eigenmanniorum (S)Cheirodon interruptus (S), Uruguay tetraGymnocharacinus bergi (P), naked characin

Order AtheriniformesFamily Atherinopsidae (A)Odontesthes hatcheri (P + CH), patagonian silverside

Order CyprinodontiformesFamily Anablepidae (W)Jenynsia lineata (S), one-sided livebearer

Family Poeciliidae (W)Cnesterodon decenmaculatus (S), ten-spotted livebearer

(McDowall and Nakaya 1987; McDowall 1988). Onespecies of Atherinopsidae, Odontesthes hatcheri, isfound in Patagonian lakes and rivers of Argentina andChile, and north throughout the central Andean regionof Argentina (Dyer 1998). Two species of Siluriformsof the family Trichomycteridae (Trichomycterus

areolatus and Hatcheria macraei) are found inPatagonia and north throughout the central Andeanregion of Argentina (Arratia 1983, 1987; Arratia andMenu Marque 1981).

Three species of Siluriforms of the familyDiplomystidae are found exclusively in Argentina,

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two of which (Diplomystes mesembrinus andD. viedmensis) are found exclusively in Patagonia(Azpelicueta 1994a, b). There are four species ofthe family Percichthydae in Patagonia. One of them,Percichhtys trucha, is found in Argentinean Patagoniaand Chile and the other three species (P. colhuapensis,P. altispinnis and P. vinciguerrai) are found exclusivelyin Argentinean Patagonia.

Finally, the most restricted distribution of all fishtaxa in the region is that of the naked characin,Gymnocharacinus bergi, limited to the thermal head-waters of a 100-km long endorheic stream, the ArroyoValcheta (Figure 1), in the Rıo Negro Province(Ortubay and Cussac 2000). This is the only fish inArgentina that is listed in the red book of species, clas-sified as endangered (Baillie and Groombridge 1996).

In summary, lakes and rivers of Patagonia are inhab-ited by 20 native species of fish, 4 of which have wideSouth American distribution, 2 have a circumpolar dis-tribution, 8 are found in Patagonia, with distributionextending northwest towards central Chile, and 6 arefound exclusively in Patagonia. Of all these species,only the patagonian silverside, Odonthestes hatcheri,and the perches of the family Percichthydae have somevalue for sport fishing.

Introduced fishThirteen species have been introduced into Patagonia,10 of which have established self-sustaining popula-tions (Table 1). Of those, 7 are salmonids, 3 of thembeing widely distributed: rainbow trout (Oncorhynchusmykiss), brown trout (Salmo trutta), and brook trout(Salvelinus fontinalis) and the remaining 4 are onlylocally abundant. The 3 non-salmonid species (Table 1)have restricted distributions.

Salmonid introductions started early in the twentiethcentury, when the Federal Government initiated anaggressive importation program from the US andEngland to populate basins throughout the regionwith ‘valuable’ sport fish (Marini 1936; Tulian 1908;MacCrimmon 1971). By the 1930s, salmonid produc-tion had been centered at the Bariloche Hatchery inNorthern Patagonia, which became the main focusof salmonid propagation in Argentina. By that time,salmonids were already well established throughoutthe region, except in Tierra del Fuego, where officialattempts to introduce them took place later, throughoutthe 1930s and 1940s. By the 1980s, all five provincesof Patagonia had their own hatcheries, continuing thespread of salmonids up to the present.

A new wave of exotic salmonid imports for netpen aquaculture is now taking place. Beginning in the1980s, salmon marine production in Chile grew dra-matically, from 53 metric tons in 1981 to 300,000 met-ric tons in 2000 (SERNAP 1996–1999; Anonymous2001). As salmon production increased, so did reportsof fish escaping from net pens and straying into therivers of southern Chile and Argentina. In recent years,we have been detecting anadromous chinook salmonspawning in headwaters of Pacific basins in Argentina(M. Pascual, unpub. data).

Present-day distribution of native andexotic fish species

We created distribution maps for all native and exoticfish species in each of the 32 major river basins ofArgentinean Patagonia. For practicality, we did notconsider small streams or lakes, limiting our focuson medium to large-size water bodies. The databaseencompasses a total of 8810 km of rivers and a totalof 10,656 km2 of lakes. We started by dividing riverbasins into strata. Each stratum consists of a single lakeor reservoir (average area 120 km2), or a physicallydistinct section of a river (average length 113.3 km),defined as a free-flowing portion of a river limitedupriver and downriver by either a lake, a reservoir, adam, or a major tributary. This partitioning schemeresulted in 148 strata (Table 2).

We then built a database of presence/absence data forall strata and all species in Table 1. A code was assignedto each species at each stratum to represent confirmedpresence (1), confirmed absence (0), or unknown status(−99). In order to do this, we circulated the databaseamong the authors repeatedly and completed as muchof it as possible. Because the available literature inrefered journals is scarce, we rely heavily on gray liter-ature, unpublished reports, personal communicationsand, in many cases, our own fieldwork experience.We also consulted several colleagues with recognized

Table 2. Distribution of 148 freshwater strata defined in ourdatabase of Patagonia by type and province (see Figure 1).

Province Rivers Lakes Reservoirs

Neuquen 14 20 1Shared Neuquen/Rio Negro 4 5Rıo Negro 6 15Chubut 18 19 2Santa Cruz 17 14Tierra del Fuego 8 5

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field experience at particular sites. Confirmed absenceswere assigned only to those strata where multi-yearmonitoring surveys based on the use of multiple sam-pling techniques had failed to record the presence of aspecies. The database was built in Access (MicrosoftCorp., US), and queries were used to extract generaldistribution patterns of native and exotic species.

To visualize distribution patterns from the database,we built a geographic information system (GIS) usingArcView GIS 3.2 (ESRI Inc., US). Presence/absencevalues were assigned to strata in the maps with eitherpolygons to represent lakes or arcs to represent sectionsof rivers. In order to characterize gradients in speciesdistributions, each stratum was assigned a centrallocation, given by its mean latitude and longitude.

In order to show comprehensive distribution mapson a regional scale for this paper, we defined largerscale strata, consisting of groups of strata sub-basins.To each of these larger strata, 70 in total, we assigned apresence if the species was present in at least one of theoriginal composing strata, an absence if the species wasabsent from all original strata, or an unknown status ifthe original strata contained all unknowns.

Literature review

We reviewed 298 published papers about Patagoniancontinental fish (Ferriz et al. 1998). We character-ized the focus of the papers, looked into the type ofresearch approach selected, determined the type ofinteraction between exotic and native species reported,and looked at evidence for impacts on the receivingcommunity.

We also reviewed 36 papers dealing with the inter-action between exotic salmonids and native freshwaterfish in Australia and New Zealand. We looked, in par-ticular, for the research approaches used and whetherthey were successful at characterizing the type of inter-action taking place between native and exotics, and atgauging the ensuing impact on indigenous fish.

Results

Species distribution

A most disturbing feature readily emerges from ourspecies-distribution database, which is the large pro-portion of strata for which we could not assign apresence/absence value (Table 3). The uncertainty

Table 3. Presence/absence data of native and exotic species in ourdatabase. For each species, the number of strata (out of a total of148) where the species is present, absent, and where the status ofthe species is still unknown are shown

Number of strata

Present Absent Unknown

Native speciesPercichthys sp. 76 28 44Galaxias maculatus 59 25 64Galaxias platei 50 16 82Odontesthes hatcheri 45 43 60Diplomystes sp. 39 52 57Hatcheria macraei 24 46 78Jenynsia lineata 9 138 1Aplochiton sp. 3 58 87Astianax eigenmannioruum 2 146Cheirodon interruptus 2 146Cnesterodon decenmaculatus 2 135 11Gymnocharacinus bergi 1 147

Exotic speciesOncorhynchus mykiss 121 7 20Salmo trutta 100 21 27Salvelinus fontinalis 75 46 27Salmo salar 17 94 37Oncorhynchus tshawytscha 6 129 13Salvelinus namaycush 5 134 9Odontesthes bonariensis 4 140 4Oncorhynchus masou 3 139 6Cyprinus carpio 2 146Corydoras sp. 2 146

is particularly abundant for native fish, where formost species there are more information voids thanconfirmed status. Information for all Galaxiids and forSiluriforms of the genus Diplomystes and Hatcheriaare particularly deficient, with over 30% and as muchas 60% of strata for which a presence/absence statuscould not be assigned. Albeit substantial, uncertaintiesabout the distribution of exotic fish are significantlysmaller (Table 3).

Uncertainty about species status varies regionallyand with habitat type (Figure 2). In general, very fewstrata have low uncertainty levels (i.e. less than 5 out of38 species with unknown status) and for only a handfulthere is no uncertainty (i.e. zero species with unknownstatus). This corresponds to the few locations wherelong-term surveys have been conducted. On average,uncertainties are greater for rivers than for lakes andreservoirs. The best information corresponds to lakesand reservoirs in the Rıo Negro Province, while thegreatest uncertainties correspond to lakes and reser-voirs in the Santa Cruz Province, as well as to riversin northern Santa Cruz. Some of these strata present as

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Figure 2. The number of species (of a total of 38) with unknownstatus along the latitudinal gradient of Patagonia. Symbols indicatethe province where a stratum is located. The upper panel shows thedata corresponding to the 81 lake and reservoir strata and the lowerpanel shows those corresponding to the 67 river strata.

many as 23 species (61%) for which status could notbe established.

The confirmed presence of native and exotic speciesallows us to build distribution maps (Figures 3 and 4).Because of the significant uncertainties in species sta-tus, these maps are likely to change considerably asmore information is gathered and should be regardedonly as preliminary. Nevertheless, they provide a gen-eral portrait of the distribution of some species. Themost widely distributed native fish are Percichthyds,Galaxiids and the Patagonian silverside, Odontestheshatcheri (Figure 3). On the other side, the nakedcharacin, Gymnocharacinus bergi, is restricted to asingle endorheic stream (Figure 3D). The most widelydistributed exotic fishes are rainbow (Oncorhynchusmykiss) and brown trout (Salmo trutta), with confirmedpresences in 82% and 68% of all investigated strata,respectively (Figure 4 and Table 3), and both withpan Patagonian distributions. Brook trout (Salvelinusfontinalis) are also widespread (51% of all strata inves-tigated, Figure 4C), while the remaining salmonidshave restricted distributions (Figure 4D).

Because of the large information gaps in ourdatabase, apparent simpatry between native and exoticfish will underestimate true simpatry. Yet, we foundwidely overlapping distributions at the spatial scale ofour analysis. Most indigenous fish are found to coexistin most strata with at least two, and most commonlywith three, exotic species (Table 4). In only 6 strata,indigenous fish were reported with no co-occurringexotic fish. These strata, however, correspond to areaswhich are so poorly surveyed and contain so manyunknowns for introduced fish that we believe there isa low probability that they constitute true ‘sanctuaries’for indigenous fish. In general, the four most conspicu-ous native species are found in simpatry with virtuallyall exotic species, while the three most conspicuousexotic fish have a large overlap at the geographic scalewith all native species.

The interaction between native andexotic fish in Patagonia and Australasia

After analyzing how deficient the available infor-mation on species presence/absence is, it shouldcome as no surprise that documentation on theimpacts that salmonids have on native communitiesof Patagonia is scarce and largely inconclusive. Thefew reports available from those who witnessed theinitial stages of salmonid introductions are based onlargely circumstantial evidence and present contrast-ing views on how abundant native fish were prior tothe introductions (Marini 1936; Gonzalez Regalado1945).

Modern literature on Patagonian fish is relativelylittle, largely concentrated on systematics and biol-ogy of native fish (some 175 references) and on thebiology and demography of salmonids (about 105 ref-erences). Only 15 papers, all non-experimental fieldstudies published after 1985, looked specifically at theinteraction between native and exotic fish. Most ofthem examined fish communities in individual lakesand reservoirs of northern Patagonia. A handful ofpapers attempted to identify community structuringprocesses by looking for contrasts in species compo-sition, microhabitat use and trophic relationships incollections of lakes.

The main results of these papers can be summarizedas follows. Salmonids feed heavily on some nativefish, particularly on Galaxiids (both gen Aplochitonand Galaxias) and silversides (Macchi et al. 1999),with large brown trout being the most piscivorous

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Figure 3. Distribution of the most endemic indigenous fish species. A: Percichthys sp. and Aplochiton sp.; B: Galaxias platei and G. maculatus;C: Diplomistes mesembrinus and D. viedmensis; D: Odonthestes hatchery and Gymnocharacinus bergi.

species. Yet, most native species show some degreeof piscivory (Ferriz 1993–1994; Bello et al. 1991;Macchi et al. 1999), particularly native perches, whichare still very abundant and even dominant over trout inmany lakes and rivers in the region.

Most studies found some degree of segregationbetween native and introduced species, either trophic(Ferriz 1993–1994; Ferriz and Salas Aramburu 1994;Grosman 1993–1994; Macchi et al. 1999; Viglianoet al. 2000), reproductive (Cussac et al. 1997), or in

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Figure 4. Distribution of major groups of exotic fish. A: rainbow trout, Oncorhynchus mykiss; B: brown trout, Salmo trutta; C: brook trout,Salvelinus fontinalis; D: Atlantic salmon, Salmo salar, chinook salmon, Oncorhynchus tshawytscha, and lake trout, Salvelinus namaycush.

habitat use (Vigliano et al. 2000). Whether segrega-tion provides a mechanism to alleviate competition andpredation or is itself the result of species interactionsremains a matter of speculation.

Only one study provides conclusive evidence of anecological disruption resulting from exotic species.In Arroyo Valcheta, a spring creek, rainbow troutprey heavily on the listed and strictly endemic naked

characin, confining it to warm headwaters (Ortubayet al. 1997; Ortubay and Cussac 2000).

The same species of salmonids found in Patagoniawere introduced into Australia and New Zealand atabout the same time. Brown and rainbow trout arethe most widespread exotic salmonids in Australasia(McDowall 1990). In contrast, brook trout, whichis widespread in Patagonia, is restricted to a few

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Table 4. Number of strata where indigenous species are known toco-occur with a different number of exotic species.

Species Number of strata where itcoexists with N exotic species

0 1 2 3 4 5

Percichthys sp. 1 7 16 35 16 1Galaxias maculatus 1 11 38 9Galaxias platei 5 4 9 24 8Odontesthes hatcheri 9 11 16 8 1Diplomystes sp. 3 8 22 6Hatcheria macraei 1 5 10 8Jenynsia lineata 2 3 2 2Aplochiton sp. 1 1 1Astianax eigenmannioruum 1 1Cheirodon interruptus 1 1Cnesterodon decenmaculatus 1 1Gymnocharacinus bergi 1

localities in New Zealand (McDowall 1968) andAustralia (McKay 1984). Unlike Patagonia, studies inAustralasia were largely conducted in rivers, many ofthem small streams; as in Patagonia, opinions aboutthe extent of the effects that trout have on native fish inNew Zealand are contradictory (McIntosh 2000).

Significant diet overlap exists between native andintroduced species, and spatial-distribution patternsconsistent with strong interaction have been docu-mented (reviewed in Crowl and Townsend 1992).Native fish in Australasian rivers have fragmented dis-tributions, highly segregated from those of brown andrainbow trout (McDowall 1968; Cadwallader 1978;Fulton 1978; Main et al. 1985), with an almost com-plete lack of co-occurrence at the level of individualsample sites (Minns 1990; Townsend and Crowl 1991).Available information suggests that once salmonidsare introduced, their impacts may be severe and rapid(Crowl et al. 1992). A manipulative field experiment,where brown trout were introduced into a section ofa New Zealand stream, resulted in a dramatic declinein abundance and condition of G. olidus within a fourmonth period (Fletcher 1979).

As far as we can tell, in no instance was competi-tion demonstrated as a critically important mechanismfor the interaction between native and exotic fish, butthere are multiple evidences of significant predation onnative species by exotic fish (reviewed in Crowl et al.1992). Field studies and a few laboratory experimentsshowed that, as in Patagonia, large brown trout are themost conspicuous piscivores in New Zealand. Studiesbased on historical data suggest that rainbow trout canalso have a strong predatory effect, being apparently

responsible for the demise of Galaxias maculatus inLake Purrumbete, Australia (Cadwallader and Eden1982).

Discussion

A logical first step for research on Patagonian freshwa-ter fish is to build up the information available on dis-tribution of native and exotic species. This kind of datawill facilitate producing workable hypotheses aboutthe interaction between native and exotic species andmay also help to identify conservation-oriented actions,even before underlying mechanisms are understood.The database we present in this paper is intended asthe foundation for such inventory work.

Our analysis revealed what some of the most notori-ous deficiencies in our current knowledge are. It shows,for example, that data on some of the most conspicuouspreys of trout (Galaxiids and Siluriforms) are particu-larly incomplete. It also indicates that uncertainties aregreater for rivers than for lakes, and that some areasof Patagonia are particularly data-deficient, such asthe north of Santa Cruz Province. Meanwhile, datafor northern Patagonia, particularly for lakes of theRıo Negro Province, are significantly better.

The literature about New Zealand rivers indicatesthat segregation between native and exotic speciesoccurs on a much smaller scale than that of ourdatabase. More specific analyses on the interactionbetween trout and natives will require looking ata smaller scale, a type of research virtually non-existent for Patagonian rivers. Also, while we foundno ‘sanctuaries’ for native species at the scale ofour database, smaller rivers are more likely to pro-vide trout-free sites and should be considered as partof future research. For example, small and unstablestreams, which are unsuitable for brown and rainbowtrout, have been found to provide refuge to some nativespecies in New Zealand (McIntosh 2000).

To understand attitudes towards exotic and nativefish in Patagonia, it is necessary to bear in mind thatsalmonid introductions preceded the strongest influxof European immigration to the region. Exotic fishare not really regarded any differently from indigenousfish by the general public and by most policymakers.Therefore, although management goals for Patagonianfish communities have not been established explic-itly, they effectively encompass: (a) preserving world-renowned trout sport fisheries, the dominating and

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exclusive focus of provincial fisheries administrations,and (b) preserving native biodiversity, the guiding prin-ciple of the National Parks Administration and a muchrelegated objective in practice. Even when there aregood reasons to believe that salmonids have had signifi-cant effects on the native freshwater biota, the lack of anarticulate vision of how virgin communities might havelooked before introductions seriously compromises theproposal of coherent conservation goals and actions.

There are, nevertheless, some opportunities fordirect conservation actions in Patagonia at the locallevel, such as protecting critically endemic speciesthrough protected areas and active exclusion of non-native species (Saunders et al. 2002). The ArroyoValcheta, a small and manageable stream, pro-vides an excellent opportunity to protect the highlyendemic and endangered naked characin throughhabitat preservation and exclusion of rainbow trout.

At the regional level, however, protecting native fishrequires a more pragmatic approach for the preser-vation of native species. The true challenge for fishconservation at the regional level, we believe, is to findways to protect native fish, compatible with maintain-ing highly priced sport fisheries. Is this at all possible?Are there management strategies that could balancefishing quality and conservation of native species?Such an approach may not appear completely satis-factory from a conservation viewpoint, but the currentmanagement scheme simply leaves native species outof the picture.

Trout management plans typically include establish-ing catch and size limits, as well as stocking fish fromhatcheries, actions that ultimately affect the composi-tion of exotic species, as well as their population ageand size structure. For instance, the stocking of rain-bow and brown trout is regularly conducted throughoutlakes and rivers of Patagonia. How would such reg-ulations and actions ultimately affect native species?Setting up a research program to deal with this questionwould require two things. First, the conservation ofnative fish should be evaluated in concert with fish-ery management of exotic species as an integrated‘cost–benefit’ analysis, something rarely done in typi-cal invasion ecology studies. Second, research shouldbe directed at understanding whether and how partic-ular exotic species affect native species. For example,is the most piscivorous brown trout more detrimentalto native species than rainbow trout? Are the predatoryimpacts of a small population of large fish greater thanthe competition impacts of a large population of small

fish? Also, do these effects vary across habitat types?Are native species more vulnerable to exotics in lakesthan in rivers (Crowl et al. 1992)?

Answering these complex questions will no doubtdemand going beyond inventory work and simplis-tic analyses of fish distribution. Exposing the natureand extent of interactions between native and exoticfish will require implementing lab experiments, fieldexperiments based on species removal or addition,and even adaptive-management frameworks based onexperimental fishing programs (Parma et al. 1998).Opportunities for such research and actions abound inPatagonia.

Acknowledgements

This study was supported in part by research grantPICT 98, 01-04420 from ANPCyT (Argentina) toMP. We greatly thank the following individuals forsharing with us unpublished data and reports onspecies presence/absence: A. Del Valle, A. Espinos,J.P. Hualde, P. Vigliano, M.L. Baiz, M. Alonso,M.T. Bello, G. Blasetti, R. Malerba, P. DeCarli, andM.A. Casalinuovo. We would like to thank DiegoVazquez, Roxana Aragon, Jim Carlton, Pablo Viglianoand two anonymous reviewers for their extremely help-ful comments on an earlier version of the manuscript.

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